1. Field of the Invention
The present invention relates to an optical element transparent to light, optical unit, and image-taking apparatus which acquires image data by making subject light form an image.
2. Description of the Related Art
Known variable-focal-length lenses include liquid crystal lenses which vary their focal length using electro-optical effect of liquid crystals. For example, Japanese Patent Laid-Open Nos. 2001-272646 and 2004-4616 describe a liquid crystal lens composed of two optically transparent substrates, a liquid crystal layer encapsulated between the two optically transparent substrates, and an electrode which applies a electric field to the liquid crystal layer. With this liquid crystal lens, the orientation of liquid crystal molecules changes with the intensity of the applied electric field, changing the refractive index of the liquid crystal lens and thereby adjusting the focal length of the liquid crystal lens.
In addition to the above liquid crystal lens, also known is a liquid lens which varies its focal length by changing surface shape of a conductive fluid by application of voltages to the conductive fluid. For example, an article “Philips' Fluid Lenses” (Mar. 03, 2004, Royal Philips Electronics) found at URL <http://www.dpreview.com/news/0403/04030302philipsfluidlens.asp> on Mar. 31, 2004 describes a liquid lens composed of a tube whose inner wall is covered by a water-repellent coating, a water-based conductive liquid and non-conductive oil encapsulated in the tube, and an electrode which applies an electric field to the water-based liquid in the tube. With this liquid lens, when no voltage is applied to the conductive water-based liquid, the water-based liquid is a hemispherical mass and an interface between the water-based liquid and oil is convex. The interface changes its shape from convex to concave according to the intensity of the electric field applied to the conductive water-based liquid. This changes the curvature radius of the lens, allowing the focal length to be changed freely.
The technique proposed in Japanese Patent Laid-Open No. 2001-272646 changes the focal length of the liquid crystal lens using the difference Δn(nψ−n⊥) between the refractive index (nψ) of liquid crystal molecules along the major axis and refractive index (n⊥) of the liquid crystal molecules along the minor axis. However, the difference Δn is too small to freely change the refractivity of the lens.
If the lens center can be shifted by changing the refractive index and surface shape, the direction of light exiting the lens can be adjusted. Thus, by mounting such a lens on a camera or the like, it is possible to prevent camera shake when shooting a subject. With the lenses described in Japanese Patent Laid-Open Nos. 2001-272646 and 2004-4616 and the article “Philips' Fluid Lenses,” in order to shift the lens center, it is conceivable to place multiple electrodes in the lenses, connect a drive line to each of the electrodes to supply voltage, and separately control the voltages applied from the multiple electrodes. However, in order to control the shifting of the lens center finely, it is necessary to install a large number of electrodes and drive lines. Consequently, with a drive method which does not employ transistors, it is difficult to change the refractive index instantly because of the need to apply voltages to different locations in sequence. Furthermore, the supplied voltages concentrated near the lens develop heat in the lens, causing changes in the refractive index of the lens. On the other hand, with a drive method which employs transistors, although it is possible to change the refractive index instantly, typical transistors needs a black matrix to shield light, which reduces the aperture ratio, thereby impairing the functionality of the lens.
Incidentally, the above problems are not limited to lenses, and are true to optical elements such as parallel plates and prisms.